In macro-optics, diffraction limits our ability to manipulate light. This limit is descriptively expressed as the limit at which two points separated by a distance Δx can be resolved. The best-case diffraction limit, as calculated by Abbe, is given as

Δx = 0.61λ/n

where λ is the wavelength of the light and n is the refractive index of the medium, which is typically about 1.5. This translates to a rule-of-thumb that to resolve features you need a wavelength less than twice the dimension. We can use visible light at a green wavelength, 550 nm, to resolve features as small as 225 nm, but not very well.

The situation improves when the optical source is very near to the target medium, the so-called near-field case. None other than Hans Bethe took time out from his Manhattan Project responsibilities to publish a paper on near-field optics,[1] perhaps as a cover for his actual work. The near-field effect was put to good use in 1992 by Bell Labsscientists as a means for high resolution scanning optical microscopy (see figure).[2] What we learn from these studies is that light can be manipulated in unusual ways when passed through sub-wavelength apertures.

Figure 11 of US Patent No. 5,272,330, "Near field scanning optical microscope having a tapered waveguide," by Robert E. Betzig and Jay K. Trautman, December 21, 1993.(Via Google Patents).[3)]

Electrical engineers at Princeton University have combined a nanoscale aperture mesh and plasmons to produce an extremely efficient photovoltaic cell that traps nearly all incident light (see figure). The holes in the mesh are 175 nm in diameter, and this mesh produced a large efficiency enhancement for an organic photovoltaic cell. Large factors are welcome, since organic photovoltaics, although less expensive, are presently less efficient than their inorganic competitors.

A plasmonic cavity mesh

The holes are 175 nm in diameter, separated by 25 nm, in a 30 nm thick gold film.

The holes are 175 nm in diameter, separated by 25 nm, in a 30 nm thick gold film. The efficiency is all the more remarkable, since the mesh shades 40% of the surface area from the incident light. The mesh replaces the typical transparent electrode, indium-tin oxide, which blocks about 20% of the light.

In experiments, these cells have trapped light to an extent that an average light-coupling efficiency of 90% was obtained, with a peak of 96%.[4-5] This excellent light coupling was achieved at all incident angles and all light polarizations. Unfortunately, because of the low intrinsic efficiency of the photoactive organic layer, these cells achieved just 4.4% power conversion efficiency for a solar irradiation standard. This was still 52% higher than the equivalent cell without the mesh and an ITO front electrode.[5]

Comparison of the optical absorption of conventional ITO cells and the Princeton PlaCSH photovoltaic cells.

For scattered light, rather than direct illumination, the mesh structure has an 8% power conversion efficiency, so it would be good for many indoor energy-harvesting tasks. Another advantage of the mesh is that it has a sheet resistance of 2.2 ohms per square, which is more than four times lower than that of transparent ITO electrodes. The Princeton team believes that their wafer-scale process can be adapted for nano-imprinting of roll-to-roll material.[5]

Another approach to light trapping is being investigated by scientists at Duke University, teamed with other scientists from France and China, and they've reported their results in a recent issue of Nature.[6-8] The Duke approach also uses plasmon resonance, and it's a potentially less expensive process that doesn't require photolithography. Instead, nanoscale metalcubes are randomly scattered on a polymer layer.[7-8] Says team leader, David R. Smith, an electrical engineering professor at Duke University,

"The nanocubes are literally scattered on the gold film and we can control the properties of the material by varying the geometry of the construct... The absorptivity of large surface areas can now be controlled using this method at scales out of reach of lithography which would otherwise be required to manipulate matter at the nanometer scale."[6]

Metals alone are reflective, but the cubes function as optical analogues of patch antennas.[6-7] The absorptive properties are tailored by the thickness of the film and the size of the nanocubes, and one thing that needs to be optimized in future work is the size uniformity of the cubes.[7-8]